Game Boy Player obviously. I essentially treat like it's a separate GBA/GBC/GB console. That is, I just leave the Game Boy Player disc inside and use the Wii for GameCube stuff.

*crosses fingers at not being hit later by the Animal Crossing delete save glitch*---"Why a numeric keypad!? This is a video game controller, not a phone!!"--Angry Video Game Nerd regarding the Intellivision controller